Young’s modulus decrease after cold forming

Young’s modulus decrease after cold forming

Haleem, A.

Neve, J.J.L. (mentor)
Staufer, U. (mentor)

Mechanical, Maritime and Materials Engineering

Precision and Microsystems Engineering

2009-02-13

The start of 21st century saw an increasing level of competition among automotives. Automotive company needs to cope with the challenges of production of high quality, cheap and environment friendly cars. This becomes possible by the use of High Strength Steel (HSS), Advance High Strength Steel (AHSS) and Ultra High Strength Steel (UHSS) with thinner gauges for automotive bodies. Besides this, automotive has also to face the issue of the reduction in “Design to Market time” which plays an important role for the company to remain competitive. This issue can be best met by the production of a “first time right” product and process and becomes possible by the use of Computer Aided Engineering (CAE). In most sheet metal forming operations, springback, which is undesirable, takes place at the end of operation which results in the deviation of final geometry from the design geometry. Springback is found higher for the metal with higher strength and for the thinner gauges and this is the reason of the higher amount of springback when HSS, AHSS and UHSS with thinner gauges are used for automotive bodies. The need of prediction of springback before an actual operation is therefore emphasized so that it can be compensated in the manufacture of expensive dies and tooling. The current practise of the manufacture of these dies is trial and error process which is time consuming, labour extensive and very expensive. An advance approach, which eliminates these problems, is the use of Finite Element (FE) analysis for the prediction of springback. In most cases, the springback with FE analysis is under predicted. There are many factors which contribute to the inaccurate springback prediction. One factor responsible for the inaccurate springback prediction is the assumption of constant Young’s modulus in FE analysis. Young’s modulus, however, is found to decrease during plastic deformation. Therefore, an accurate model is needed to be implemented in FE analysis which can predict the decrease in Young’s modulus. With passage of time, Young’s modulus is found to restore to its initial value. The present study focuses on this aspect of springback prediction. Bake Hardenable (BH) steel has been selected to investigate the degradation and recovery in Young’s modulus. Two models are presented from literature in this report which are responsible for degradation when sheet metal is subjected to plastic deformation. The recovery in BH steel is considered to be due to strain ageing phenomenon in BH steel. The degradation in E modulus is found to be a function of dislocation density and average dislocation loop length between pinning points. In order to estimate the Young’s modulus as a function of prestrain, these two parameters need to be known. The decrease in Young’s modulus and its recovery has been measured experimentally by two methods i.e. the dynamic method (Impulse Excitation Technique) and a static method (Tensile Test). The results are thoroughly discussed and some conclusions are drawn from it. At the end of the report, some recommendations are suggested for the future work. The report also compares the two methods of measurement and helpful tips are given for future work using these methods of measurement.

Young's Modulus
cold forming

http://resolver.tudelft.nl/uuid:18e9acc0-c605-41d6-a5e3-a65914503e05

2010-01-15

Student theses

master thesis

(c) 2009 Haleem, A.